GPU Render takes ages "Building Dynamic Mesh trees"

[blackbox]

I am rendering some kitchen interior scenes using several GPU's, the render seems to get to the noise threshold etc within an hour ish but the next phase at 1005 of "building dynamic mesh trees is taking 2-4 hours, Anyone else come across this issue.?

I'm on win 10 Maya 2017 update 4 Vray 3.6

Any clues would be helpful, all GPU's are maxed out 100% and the CPU too as i've got hybrid mode on. It seems to do it with just GPU's too. !!

[yolov]

Would it be possible to share the scene? It will make it easier for us to figure out why it might be taking so long.